In about the last 10 years, a new concept for the insulation of high voltage substation power interrupting equipment and transmission lines, and potentially for long distance power transmission lines has been developed. The new concept involves insulating all components energized at high voltages by surrounding them with an atmosphere of pressurized, insulating sulfur hexafluoride gas. Insulating spacers and other components used in pressurized, SF.sub.6 gas insulated high voltage transmission systems must provide a very high degree of arc and track resistance even in an arced SF.sub.6 gas environment.
These spacers may be used at voltage gradients as high as 60 kV. to 120 kV. per inch. Since these spacers are used to isolate large aluminum or copper conductors from the walls of pressurized ducts, it is essential that they have linear thermal expansion coefficients closely matching those of aluminum and copper at about 22 and 18 .times. 10.sup.-6 in./in./.degree. C respectively.
Epoxy resins of the diglycidyl ether of bisphenol A type and silica fillers are known to have poor arc and track resistance in an arced SF.sub.6 environment. Luck et al., in U.S. Pat. No. 3,828,000 provided a composition with improved SF.sub.6 resistance, by using anhydride cured cycloaliphatic epoxy resins filled with alumina trihydrate and thickened with asbestos. These compositions, developed for coatings of thin cross section, tended however to have high coefficients of thermal expansion, and when cast in massive thickness in contact with metals, may in some instances exhibit cracking during severe thermal cycling.
Since one extremely desirable attribute of a power circuit breaker and high voltage transmission system is high reliability, what is needed in an anhydride cured epoxy resin system not only resistant to arced SF.sub.6 gas but also having a coefficient of linear thermal expansion close to that of aluminum and copper.